1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method and apparatus for efficiently allocating radio resources to transmit an uplink message of a terminal, or User Equipment (UE), by a network node.
2. Description of the Related Art
The Universal Mobile Telecommunication Service (UMTS) system is a 3rd Generation (3G) asynchronous mobile communication system employing Wideband Code Division Multiple Access (WCDMA) based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), both of which are European mobile communication systems. In 3rd Generation Partnership Project (3GPP) in charge of UMTS standardization, a Long Term Evolution (LTE) system is under discussion as the next generation mobile communication system of the UMTS system. The present invention will be described herein with reference to the LTE system, which will now be briefly described.
LTE is a technology for implementing packet-based communication at a high data rate of a maximum of about 100 Mbps, aiming at commercialization in around 2010. To this end, several schemes are under discussion, such as one for reducing the number of nodes located in a communication path by simplifying a configuration of the network, and another for maximally approximating radio protocols to radio channels.
FIG. 1 illustrates an Evolved UMTS mobile communication system to which the present invention is applied.
Referring to FIG. 1, an Evolved UMTS Radio Access Network (E-UTRAN or E-RAN) 110 is simplified to a 2-node configuration of Evolved Node Bs (ENBs) 120, 122, 124, 126 and 128, and anchor nodes 130 and 132. A UE 101, or terminal, accesses an Internet Protocol (IP) network by means of the E-UTRAN 110.
The ENBs 120 to 128 correspond to the existing Node B of the UMTS system, and are connected to the UE 101 over radio channels. Compared to the existing Node B, the ENBs 120 to 128 perform more complex functions. Particularly, in LTE, because all user traffic including the real-time services, such as Voice over IP (VoIP), is serviced over a shared channel, the ENB collects status information of UEs to perform scheduling depending thereon, and controls a function related to management of radio resources. In addition, control protocols, such as Radio Resource Control (RRC), are included in the ENBs 120 to 128. Generally, each ENB controls a plurality of cells.
To realize the data rate of a maximum of 100 Mbps, LTE uses Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology in a 20-MHz bandwidth. Further, the ENB performs Adaptive Modulation & Coding (AMC) that adaptively determines a modulation scheme and a channel coding rate according to channel status of the UE 101.
Like the mobile communication system supporting High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), and Enhanced Dedicated Channel (E-DCH) services, the LTE system also performs Hybrid Automatic Repeat reQuest (HARQ) between the UE 101 and the ENBs 120 to 128. Because various Quality of Service (QoS) requirements cannot be satisfied only with HARQ, Outer ARQ in the upper layer can be performed between the UE 101 and the ENBs 120 to 128.
The HARQ is a technique for soft-combining previously received data with retransmitted data without discarding the previously received data, thereby increasing the reception success rate. This is used to increase the transmission efficiency in high-speed communication such as HSDPA and EDCH.
The random access procedure to which the present invention is applied is used as a procedure between a UE and a network node, in which a UE in RRC idle mode or an RRC connected mode matches uplink timing sync with the ENB for (initial) uplink message/data transmission, sets initial uplink transmission power, and/or requests radio resource allocation for the (initial) uplink message/data transmission. For a definition of the RRC idle mode and RRC connected mode, reference can be made to the 3GPP TR25.813v700 standard.
In brief, the RRC idle mode generally refers to a state of a UE, in which the ENB has no context information for the UE and the anchor node, or upper node, has context information of the UE, so a location of the UE is managed not in units of cells but in units of tracking area for paging.
The RRC connected mode refers to a state of a UE, in which not only the anchor node but also the ENB have the context information of the UE and an RRC connection is set up between the UE and the ENB, so the location of the UE can be managed in units of cells.
FIG. 2 illustrates a conventional random access procedure in a 3GPP LTE system.
Referring to FIG. 2, reference numeral 210 denotes a UE, and reference numeral 211 denotes an ENB that controls the cell in which the UE 210 is located.
Step 221 indicates an operation in which the UE 210 triggers a random access procedure. For example, this can indicate the case where to start a call, an RRC idle mode UE (UE in the RRC idle mode) needs to transmit an uplink control message which allows the ENB 211 to acquire UE context information, set up an RRC connection between the UE 210 and the ENB 211, and transmit a service request to an anchor node.
If the random access procedure is triggered in step 221, the UE 210 randomly selects one of a total of X random access preambles agreed with the ENB 211 in step 231. Thereafter, in step 241, the UE 210 transmits the selected random access preamble to the ENB 211 over a predetermined channel/time.
When transmitting the random access preamble in step 241, the UE 210 sets initial random access preamble's transmission power of UE by applying Open Loop Power Control (OLPC). Equation (1) shows the conventional manner of performing the conventional OLPC.PTX=Lpilot+IBTS+SIRTARGET  (1)
The parameters of Equation (1) are defined as follows:                PTX: a transmission power level [dBm] of a channel DPCH;        Lpilot: a path loss [dB] estimated using a measure of a downlink pilot channel and a transmission power of a signaled pilot channel;        IBTS: an interference level that a receiver of an ENB (or Base Transceiver System (BTS)) experiences;        SIRTARGET: a target Signal-to-Interference Ratio (SIR) [dB] for maintaining the transmission quality of each UE. It can be either signaled separately for each UE or signaled commonly for all UEs.        
If the random access preamble is retransmitted due to the failure in the initial random access preamble transmission of step 241, a delta value (hereinafter power ramp step) is added to the power that is set during the previous random access preamble transmission. The power ramp step can be either signaled, or defined as a specific value.
In step 242, the ENB 211 transmits to the UE 210 a response message to the random access preamble received in step 241. The response message 242 includes such information as a random access preamble identifier indicating the random access preamble received in step 231, uplink timing sync information for matching uplink timing sync and radio resource allocation information for transmission 251 of the next uplink upper message of the UE 210.
In the transmission of the response message by the ENB 211 in step 242, the ENB 211 can perform synchronous transmission using the timing relationship determined for the transmission of step 241 by the UE 210.
If the information received in step 242 includes a random access preamble IDentifier (ID) mapped to the random access preamble transmitted in step 241 by the UE 210 itself, the UE 210 corrects the uplink transmission timing, using the uplink timing sync information included in the received information of step 242. In step 251, the UE 210 transmits the corresponding upper message over the corresponding channel/time using the allocated radio resources.
The message transmitted in step 251 can be an RRC message or a Non-Access Stratum (NAS) message. Alternative, the message can be a combined message of the RRC message and the NAS message. Here, the RRC message indicates a message for Radio Resource Control (RRC), having a UE and an ENB as protocol endpoints, and the NAS message indicates a message for controlling parameters such as mobility, service and session of a UE, having a UE and an anchor node as protocol endpoints.
However, in the 3GPP LTE system that performs the random access procedure of FIG. 2, when the ENB 211 allocates, to the UE 210, radio resources for transmission of an upper message in step 242, it can perform resource allocation only for the message size guaranteed such that all UEs in the cell can transmit the message. This is because when the ENB 211 receives the random access preamble from the UE 210 in step 241, the information transmitted through the random access preamble only includes a random ID.
In other words, the random access preambles have only the random IDs without including other information, to prevent the UE 210 from selecting the same random access preamble, thus preventing occurrence of the collision.
Therefore, because the ENB 211, receiving this random access preamble, cannot acquire any information necessary for scheduling, from the random access preamble, even though the UE is located in the cell boundary, the ENB 211 cannot allocate the radio resources for the transmission-guaranteed message size.
Therefore, the random access procedure of the mobile communication system shown in FIG. 2 is inefficient in scheduling the next message transmitted from the UE 210 by the ENB 211.
In addition, if the ENB 211 includes in the random access preambles the information (e.g., cause/type information of the random access procedure, priority information of the random access procedure and radio channel condition information) capable of assisting in performing scheduling, the ENB 211 may very efficiently perform scheduling on the next message transmitted from the UE 210.
However, the number of random access preambles that the UE can guarantee the transmission at any place in the cell is limited, using the limited radio resources when there is no RRC connection set up between the UE and the ENB.
To carry all the information on the limited random access preambles decreases the number of random IDs that reduce the collision probability, thereby causing the increasing collision problem that multiple UEs select the same random access preamble in the random access procedure, in which procedure an increase in the collision probability to at least a certain level may raise a fatal problem.
Therefore, the current mobile communication system needs an efficient random access procedure for solving the foregoing problems.